The Goldfinch may have won Donna Tartt the Pulitzer, praised by judges as a novel which “stimulates the mind and touches the heart”, but the acclaimed title’s 800-odd pages appear to have intimidated British readers, with less than half of those who downloaded it from e-bookseller Kobo making it to the end.

New data from Kobo shows that, although The Goldfinch was the 37th bestselling ebook of the year for the retailer, it was completed by just 44.4% of Kobo’s British readers. Kobo speculated that it “likely proved daunting for some due to the length of the novel”.

Twelve Years a Slave, Solomon Northup’s account from 1853 of how he was kidnapped and sold into slavery – “I sighed for liberty; but the bondsman’s chain was round me, and could not be shaken off” – was, according to Kobo, similarly overwhelming. Ninth on their British bestseller list, following the hugely successful film adaptation, the book was completed by just 28.2% of British readers.

The onset of digital reading means that Kobo – and other ebook retailers – are able to tell more than ever before about how readers engage with books: which they leave unopened, which they read to the end, and how quickly they finish.

Earlier this year, the American mathematician Jordan Ellenberg proposed the so-called “Hawking Index” in a blog for the Wall Street Journal, using Amazon’s “Popular Highlights” feature in an attempt to pinpoint how far into novels readers were actually getting, but retailers have been reluctant to share the data they are harvesting themselves. Kobo’s first analysis of trends in e-reading, released on Wednesday, reveal an unexpected divide between bestsellers, and the books that readers actually complete.

After collecting data between January and November 2014 from more than 21m users, in countries including Canada, the US, the UK, France, Italy and the Netherlands, Kobo found that its most completed book of 2014 in the UK was not a Man Booker or Baileys prize winner. Instead, readers were most keen to finish Casey Kelleher’s self-published thriller Rotten to the Core, which doesn’t even feature on the overall bestseller list – although Kelleher has gone on to win a book deal with Amazon’s UK publishing imprint Thomas & Mercer after selling nearly 150,000 copies of her three self-published novels.

“Rotten to The Core by Casey Kelleher was the most completed book in the UK, with 83% of people reading it cover to cover,” said Kobo, “whereas the number one bestselling ebook in the UK, One Cold Night by Katia Lief [also a thriller] was only completed by 69% of those who read it.”

Kobo’s UK ebook bestseller list also features novels by major names including Gillian Flynn, John Green, James Patterson and Robert Galbraith, while its “most completed” list mixes romance, thrillers and erotica by the likes of Sylvia Day, Stella Rimington, Nora Roberts and Lynda La Plante. Kobo said that thriller powerhouse Patterson “was the most completed author in the UK for his entire portfolio of books”.

“A book’s position on the bestseller list may indicate it’s bought, but that isn’t the same as it being read or finished,” said Michael Tamblyn, president and chief content officer at Kobo. “A lot of readers have multiple novels on the go at any given time, which means they may not always read one book from start to finish before jumping into the next great story. People may wait days, months, or even until the following year to finish certain titles. And many exercise that inalienable reader’s right to set down a book if it doesn’t hold their interest.”

Kobo also revealed that the people of Britain were most likely to finish a romance novel, with 62% completion, followed by crime and thrillers (61%) and fantasy (60%). Italians were also most engaged by romance (74% completion), while the French preferred mysteries, with 70% completion.